Studies are being conducted on the mechanism of action of colon specific chemical carcinogens, in particular, 1,2-dimethylhydrazine. This involves the difficult problem of obtaining the C-14 labeled chemical free of radiochemical impurities. Analytical techniques were developed which permit the separation and identification of the main metabolites of dimethylhydrazine, including monomethylhydrazine, azomethane, azoxymethane, methylazoxymethanol, and conjugates of the latter. Preliminary findings include the detection of these materials in urine and bile of rats given dimethylhydrazine. A sensitive polarographic analysis was developed for azoxymethane and methylazoxymethanol. In another approach, dimethylhydrazine was administered together with phenobarbital, chrysene, or Beta-naphthoflavone, which are known to increase the levels of metabolizing enzymes. Gross data suggest that tumors in the rectum appear to be increased by phenobarbital. For the first time, colon cancer histologically similar to that in man was induced in the guinea pig by intrarectal administration of methylnitrosourea or methylnitrosoguanidine. Transplantation experiments are underway. These data plus collateral findings that these chemicals can cause cancer in germ-free rats suggest that the colon mucosa can specifically metabolize this type of agent to active intermediates. Future work will need to establish the role, in any, of the intestinal microflora and also demonstrate directly the metabolic capability of colonic tissue.